Industrial Gas Turbines

The rotor blades ( "Ill. 3.3-1" and "Ill. 3.3-2") of modern gas turbines are precision castings with a complex inner cooling air passages and a multiplicity of openings to the surface, through which air exits for the purpose of protective film cooling. ( "Ill. 3.3-4"). The blade material is normally a nickel base alloy that hardens through a precipitation phase (Ni3 Al) i.e., functions against creep. This alloy can be present as polycrystalline, directionally solidified or as a single crystal ( "Ill. 3.3-4"), depending on the used casting process. The orientation, respectively, the avoidance of the grain boundaries, carries the price that this indeed forms an especially weak point for creep and thermal fatigue.

The blade tip can possess a shroud that is twisted against the neighboring blades to avoid vibration ( "Ill. 3.4-1") and takes over the sealing to the casing. Frequently, however, rotor blades without shroud are used in the HPT. Their tips are sealed against the casings through a rub system (similar in its function as to a compressor). For this purpose, some blades on the tips are plated in the form of brazed on hard particles. The casings are installed with so called turbine liner segments on the opposite side that function as abradables with filled or unfilled honey combs in older engines, (compare "Ill. 3.4-5"). In modern engines, dense plasma sprayed coatings out of zircon oxide ( "Ill. 3.2.3-5") are used as thermal barriers. More recently, rotor blades are equipped with ceramic thermal barrier coatings. Here, apparently vapor deposited coatings (PVD) with an especially thermal fatigue resistant columnar structure ( "Ill. 3.2.3-6") has prevailed over plasma sprayed coatings . This relates to a complicated multiple coating build up with a bond coating and / or an oxidation protection coating towards the base material ( "Ill. 3.2.3-7").

Despite the good oxidation resistance of the Ni-based alloys (superalloys) this alone is not sufficient today for the extremely long guarantee times. Therefore normally HPT blades are provided with oxidation protective coatings at least on the outer surface. Mostly they are produced by diffusion of aluminium ( "Ill. 3.3-7"). During operation the oxidation forms a protective dense Al2 O3 layer. To avoid oxidation on the inside of the blade, more and more the cooling structure is also equipped with a protective diffusion coating.